Multilayer printed circuit boards (PCB) have emerged as a solution to the problem of interconnecting miniature electronic components into complex systems. The area of application which shows multilayer PCBs to the greatest advantage is the interconnection of integrated circuits. Integrated circuits and multilayer PCBs can provide drastic reductions in the overall size and weight of a complete system. They find applications in high end electronics such as large computers, military equipment, and are increasingly found in consumer electronics. The multilayer technique makes more than two planes (normally 3-12) available to the PCB designer for running conductors and making large ground or supply areas, very useful in high speed circuitry. However, since multilayers represent the most complex type of PCB, their cost is very high, owing to the complexity of the manufacturing process and the lower production yields.
Multilayer PCBs are manufactured using a large number of steps and processes. Basically, the multilayer PCB is made by laminating several thin sublayers together. Copper clad laminates are normally used to make the etched circuitry layers, and are bonded together with prepreg or bonding sheets. To make a four layer PCB, for example, two single-sided laminates and one double-sided laminate are employed with two sheets of prepreg. The panel which forms the interior layer (the double sided laminate) is printed and etched on both sides, and locating holes are punched. A sandwich of all the panels is then made, consisting of (from bottom to top):
1) a panel with copper foil on one side, placed copper side down;
2) a sheet of prepreg;
3) the double sided etched panel;
4) another sheet of prepreg; and
5) a panel with copper foil on one side, placed copper side up.
This sandwich is inserted between the plates of a laminating press, similar to those used in the manufacture of conventional copper clad laminates, and is heated and pressed for a period of time. During lamination, the heated resin softens and the applied pressure causes it to flow and bond the panels together. Meanwhile, the polymerization reactions of the resins take place and the material hardens. The sandwich is then cooled and removed from the press. The resulting single strong panel has the two internal etched copper layers centrally embedded in the resin. The panel is then drilled, and the drilled holes are metallized to provide the electrical connections between the inner and the outer layers. The outer two layers are then etched to make the circuitry patterns. Further plating may then be taken up, to increase the thickness of the metallized holes. The resulting structure is shown in FIG. 1. Note that the two interior layers 5 are connected together by the metallized hole 7, which also further connects them to the exterior two layers 8, 9.
While this construction provides a number of advantages, it also has certain disadvantages. As shown in FIG. 1, the plated through hole 7 connects the inner layers 5 to the outer layers 8, 9. If one desires to connect the two inner layers and separately connect the two outer layers, two holes would be needed, one for each set of connections. Obviously, two holes, and the associated annular rings, will take up more space on the PCB surface than one hole. Artisans have sought to use ever decreasing hole sizes in order to circumvent the need for additional real estate on the board surface when making these conventional connections. But small hole diameters increase the difficulty of plating metal in the holes, creating a trade-off between hole diameter (cost) and board size. Further, the lamination step is critical, because any lamination defects will result in breaks or fractures in the hole metallization, creating open circuits. There is no method of checking the quality of the connection between the hole plating and the internal layers, except by destructive microsectioning. Repairability of multilayers is also very low, except for defects on the external layers. Clearly, a need exists for a better, more efficient way to couple the inner layers and the outer layers in separate sets.